If your rifle is the G-Spec version the stock barrel will indeed be brass. If your rifle is the Pro Sniper / Real Shock (no silencer w/ iron sights) then the stock barrel will be made of aluminum. The easiest way to tell the difference between stock barrels and the aftermarket upgraded ones is the muzzle end of the barrel. The stock barrels have little to no bevel on them whereas the upgraded ones have a very pronounced bevelled edge on them.
Stock on the left, upgraded on the right. (Picture source: 888's VSR-10 guide)